1. Field of the Invention
This invention relates to a method for producing an electrical conductor which is required to have a small diameter and a high quality, such for winding copper coils used as a magnet wire, a conductor used in acoustic nd image-forming appliances such as stereos or VTRs, and a bonding wire for connecting semiconductor elements and transistors for example in integrated circuits.
2. Description of the Prior Art
Conventionally, in producing thin metal wires, first an ingot is prepared. The ingot is then subjected to repeated multistage hot and cold workind until a thin wire of the desired size is obtained.
As a variation thereof, there is a dip forming process as described, for example, on pages 980-987 of Report No. 12, Vol. 21, (1982) from the Japan Institute of Metals. In this process, a core wire is passed at high speed through a crucible containing molten copper, whereby said core wire is thickened as the molten copper sticking thereto solidifies. Thereafter, the core material is rolled to produce a roughly drawn wire.
The conventional methods described above were each developed as suitable methods for mass production (for example, several tons/hour to tens of tons/hour). Thus, they cannot always be said to be suitable methods, from the standpoint of quality, quality control, and processability, for producing wires such as conductors used for modern electronics which requires high purity, high quality and severe thinning of the wire.
That is, when metal is melted in large quantities, it is impossible to prevent contamination by foreign matter. Further, hot working often causes foreign matter to enter in the vicinity of the metal surface. In the field of very thin wires, wire breakage forms a major factor which impedes productivity. It has been found that most of the wire breaks are caused by foreign matter such as ceramics and iron powders contaminating into the metal during melting nd casting or working. Further, a multistage wire drawing proces complicates the control of lubricants, dies, and wire drawing machines.
For conductors used for wiring acoustic and image-forming appliances such as stereos and VTRs, it is desirable that conductors contain a minimum of dissimilar elements including such components as oxygen. However, it is difficult to make the entire conductor of high purity copper. The reson is that if, for example, an ingot of high purity copper is remelted or cast, dissimilar elements contaminate or foreign matter often contaminates into the metal during processing, thus making it impossible to maintain the required high purity. This is so because the use of conventional conductors for acoustic and image-forming appliances has made it possible to obtain sounds or images of good quality.
It is required that bonding wires for connecting semiconductor elements be small in diameter, reliable and high in quality. Conventionally, as for this type of bonding wires, thin wires of gold, aluminum or copper have been used in practice or investigated for practical use. In the case of a thin wire of gold, its connecting property is good, but there is a problem of high cost. In recent years, to reduce cost by avoiding the use of noble metals, bonding wires of aluminum, aluminum alloy, copper or copper alloy have received attention to see whether such wires could be put into practical use. In the case of a bonding wire to be used for connection, contamination by foreign matter should be avoided to maintain the good connecting property. That is, it is considered desirable to use high purity metal in making bonding wires.
However, high purity metal itself is of high cost. Further, its drawing property is poor because of its lack of strength. For example, in the case of a bonding wire of high purity aluminum, because of its low recrystallization temperature, it is recrystallized by the heat of friction produced during drawing. Further, if an ingot for bonding wires made of high purity metal is remelted and cast, there is the danger of foreign matter contaminating into the metal during processing, thus making it impossible to maintain the required high purity.